cc: Introduce BeginFrame sequence numbers and acknowledgements.

services/ui/public/cpp/window_compositor_frame_sink.cc

Index: services/ui/public/cpp/window_compositor_frame_sink.cc
diff --git a/services/ui/public/cpp/window_compositor_frame_sink.cc b/services/ui/public/cpp/window_compositor_frame_sink.cc
index 2d893733bc679977bfcb12a1fc7e36f8cc5ca5be..4cb1017678f45469e8dbc2e0fc83c1c40e45f2d1 100644
--- a/services/ui/public/cpp/window_compositor_frame_sink.cc
+++ b/services/ui/public/cpp/window_compositor_frame_sink.cc
@@ -5,6 +5,7 @@
 #include "services/ui/public/cpp/window_compositor_frame_sink.h"
 
 #include "base/bind.h"
+#include "cc/output/begin_frame_args.h"
 #include "cc/output/compositor_frame.h"
 #include "cc/output/compositor_frame_sink_client.h"
 #include "gpu/ipc/client/gpu_channel_host.h"
@@ -104,6 +105,12 @@ void WindowCompositorFrameSink::OnBeginFrame(
   begin_frame_source_->OnBeginFrame(begin_frame_args);
 }
 
+void WindowCompositorFrameSink::OnDidFinishFrame(const cc::BeginFrameAck& ack) {
+  // If there were updates, SubmitCompositorFrame serves as ack.
+  if (!ack.has_updates)
+    compositor_frame_sink_->BeginFrameDidNotDraw(ack);
+}
+
 void WindowCompositorFrameSink::ReclaimResources(
     const cc::ReturnedResourceArray& resources) {
   DCHECK(thread_checker_);